Many devices and procedures have been developed for reacting or treating ores and other metallic compounds in plasma reactors in which the plasma may be generated by radio-frequency induction or by striking an arc between two electrodes. In both types of plasma reactors the ore or any other materials composing the reactant charge or feed are reacted or treated by entraining them in the plasma gas within the reactor for as long as possible in order to expose the materials to the intense heat for a sufficient period of time. Since the reacting materials are in a suspended state, a substantial residence time in the plasma reactor's extremely hot environment is required to ensure that the reacting materials contact each other so that the desired reaction will occur to a reasonable degree. Besides the difficulty with attaining adequate residence times, the plasma arc reactors may exhibit anode erosion caused by the severe conditions existing at the point of attachment of the electric arc to the anode. With the reactants suspended in the plasma between the electrodes, the arc directly impinges upon the anode eroding it.
A method and apparatus are described in U.S. Pat. No. 4,002,466 for obviating the problem of anode erosion and for providing the reactants with an extended residence time and intimate contact within the plasma reactor. A plasma arc torch is disclosed which incorporates a swirling vortical stabilizing gas stream within a reaction chamber formed by an anode tube. Reactant particles introduced between the ends of the anode are entrained in the vortex. When an arc is struck to generate the plasma, sufficient heat is afforded to melt the particles into a falling-film of material on the wall of the anode.
In the device of the patent the electric arc no longer directly impinges the anode wall but rather attaches via the film of material coating the anode wall. The falling film thus acts as a protective as well as a thermally insulating coating on the anode tube. Furthermore, the vortically swirling gas stream stabilizes the location of the arc attachment to the falling-film.
However, a serious problem arises during the reaction of certain compounds in such falling-film plasma arc reactors and other transferred plasma arc reactors.
By transferred plasma arc reactor we mean a plasma arc reactor in which the electrical arc stabilizes between an electrode (cathode) and the workpiece which is connected in a circuit as the other electrode (anode). A transferred plasma arc can be created in two ways. First, a pilot plasma arc can be struck between a cathode and an anode in a plasma reactor which has the working (stabilizing) gas fed under a high velocity between the electrodes to exit out an opening. Situated in close proximity to this opening is a workpiece that is connected into the electrical circuitry such that it too is an anode. The flow rate of the working gas through the plasma reactor is increased to the point where the electrical arc is actually blown down from the reactor anode to attach to the workpiece anode. The electrical arc and the plasma stream now extend from the cathode within the reactor to the workpiece.
A common embodiment of this type of transferred plasma arc furnace comprises an electrode positioned in the bottom of a crucible or containing vessel which holds a layer of melt or solid scrap to be melted by the plasma. The plasma arc torch is disposed apart from the containing vessel. In this embodiment, which is shown as FIG. 1 and is described in a subsequent part of this specification, the electrical arc is blown down to transfer and attach to a workpiece anode via the melt or solid scrap in the vessel.
Therefore, we define a transferred plasma arc reactor as being a plasma arc reactor in which the electrical arc emanating from one electrode attaches to a reaction layer covering a second electrode. The reaction layer may comprise the charged reactants solely or also include reaction products.
The second method of creating a transferred plasma arc is one in which the electrical arc is caused to attach to the film on the anode tube in a falling film reactor. It is obvious that such a falling film plasma arc reactor comes within the definition of a transferred plasma arc reactor set forth above.
The problem associated with such transferred arc reactors, as defined above, can be explained by way of the following example. The decomposition reaction of molybdenum disulfide (MoS.sub.2) to produce metallic molybdenum was attempted utilizing a falling-film plasma reactor. The molybdenum disulfide was fed to the reactor as is known in the art but the reaction did not proceed. Almost no product was formed and the throat section of the anode above the ore injection ports and falling film was badly eroded. This erosion was apparently caused by a wandering electrical arc.
Accordingly, there is a need for a method of controlling the arc in a transferred plasma arc reactor.
There is a need for a method of preventing the erosion of the anode of a falling film, transferred plasma arc reactor caused by a wandering electrical arc.
There is a further need for a method of producing molybdenum from molybdenum disulfide in a plasma arc reactor.